Vitamin D refers to a group of fat-soluble secosteriods that the human body can synthesize through adequate exposure to sunlight. More specifically, vitamin D3 is made in the skin when 7-dehydrocholesterol reacts with ultraviolet B (“UVB”) light. Vitamin D can also be absorbed from the various dietary sources, such as fatty fish (e.g., salmon and tuna), vitamin D fortified foods (e.g., dairy and juice products), and vitamin D supplements. Once absorbed, the vitamin D travels through the bloodstream to the liver where it is converted into the prohormone calcidiol. The calcidiol is, in turn, converted into calcitriol (the hormonally active form of vitamin D) by the kidneys or monocyte-macrophages in the immune system. When synthesized by the monocyte-macrophages, calcitriol acts locally as a cytokine to defend the body against microbial invaders. Kidney-synthesized calcitriol circulates through the body to regulate the concentration of calcium and phosphate in the bloodstream, and thereby promotes adequate mineralization, growth, and reconstruction of the bones. Therefore, an inadequate level of vitamin D, (typically characterized by a calcidiol concentration in the blood of less than 20-40 ng/m2) can cause various bone softening diseases, such as rickets in children and osteomalacia in adults. Vitamin D deficiency has also been linked to numerous other diseases and disorders, such as depression, heart disease, gout, autoimmune disorders, and a variety of different cancers.
Recently, vitamin D deficiency has become a prominent condition due, at least in part, to increasingly metropolitan populations and the resultant indoor lifestyles that inhibit adequate daily exposure to sunlight for vitamin D production. The growing emphasis on skin cancer awareness and sunscreen protection, which blocks UVB rays, may have also increased the spread of vitamin D deficiency. Additionally, various environmental factors, such as geographic latitude, seasons, and smog, further impede sufficient vitamin D production.
Physicians have recommended vitamin D supplements as a preventative measure to increase vitamin D levels. The American Institute of Medicine, for example, recommends a daily dietary vitamin D intake of 600 international units (IU) for those 1-70 years of age, and 800 IU for those 71 years of age and older. Other institutions have recommended both higher and lower daily vitamin D doses. The limitations on daily dosages also reflect an effort to prevent ingesting too much vitamin D, which can eventually become toxic. In contrast, the human physiology has adapted to significantly higher daily doses of vitamin D from sunlight (e.g., 4,000-20,000 IU/day or more). UVB radiation has been identified as a more desirable source of vitamin D because of the ease at which vitamin D is produced from exposure to sunlight and the body's natural ability to inhibit excessive vitamin D intake through the skin.